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MATH 248A. the CHARACTER GROUP of Q 1. Introduction The MATH 248A. THE CHARACTER GROUP OF Q KEITH CONRAD 1. Introduction The characters of a finite abelian group G are the homomorphisms from G to the unit circle S1 = fz 2 C : jzj = 1g. Two characters can be multiplied pointwise to define a new character, and under this operation the set of characters of G forms an abelian group, with identity element the trivial character, which sends each g 2 G to 1. Characters of finite abelian groups are important, for example, as a tool in estimating the number of solutions to equations over finite fields. [3, Chapters 8, 10]. The extension of the notion of a character to nonabelian or infinite groups is essential to many areas of mathematics, in the context of harmonic analy- sis or representation theory, but here we will focus on discussing characters on one of the simplest infinite abelian groups, the rational numbers Q. This is a special case of a situation that is well-known in algebraic number theory, but all references I could find in the literature are based on [4], where one can't readily isolate the examination of the character group of Q without assuming algebraic number theory and Fourier analysis on locally compact abelian groups. (In [2, Chapter 3, x1], the determination of the characters of Q is made without algebraic number theory, but the Pontryagin duality theorem from Fourier analysis is used at the end.) The prerequisites for the discussion here are more elementary: familiarity with the complex exponen- tial function, the p-adic numbers Qp, and a few facts about abelian groups. In particular, this discussion should be suitable for someone who has just learned about the p-adic numbers and wants to see how they can arise in answering a basic type of question about the rational numbers. Concerning notation, r and s will denote rational numbers, p and q will denote prime numbers, and x and y will denote real or p-adic numbers (depending on the context). The word \homomorphism" will always mean \group homomorphism", although sometimes we will add the word \group" for emphasis. The sets Q, R, Qp, and the p-adic integers Zp, will be regarded primarily as additive groups, with multiplication on these sets being used as a tool in the study of the additive structure. 2. Definition and Examples The definition of characters for finite abelian groups makes sense for any group. 1 2 KEITH CONRAD Definition. A character of Q is a group homomorphism Q ! S1. As in the case of characters of finite abelian groups, under pointwise multiplication the characters of Q form an abelian group, which we denote by Qb . Examples of nontrivial characters of Q are the homomorphisms r 7! eir and r 7! e2πir. Our goal is to write down all elements of Qb using explicit functions. The usefulness of Qb can't be discussed here, but hopefully the reader can regard its determination as an interesting problem, especially since the end result will be more complicated than it may seem at first glance. The prototype for our task is the classification of continuous group ho- momorphisms from R to S1. The example x 7! eix is typical. In general, for any y 2 R we have the homomorphism x 7! e2πixy: The use of the scaling factor 2π here is prominent in number theory, since it makes certain formulas much cleaner. For the reader who is interested in a proof that these are all the continuous homomorphisms R ! S1, see the appendix. We will not need to know that every continuous homomor- phism R ! S1 has this form, but we want to analyze Qb with a similar goal in mind, namely to find a relatively concrete method of writing down the homomorphisms Q ! S1. Keep in mind that we are imposing no continu- ity conditions on the elements of Qb ; as functions from Q to S1, they are merely group homomorphisms. (Or think of Q as a discrete group, so group homomorphisms are always continuous.) Let's now give examples of nontrivial characters of Q. First we will think of Q as lying in the real numbers. For nonzero y 2 R, the map x 7! e2πixy is continuous on R, hits all of S1, and Q is dense in R, so the restriction of this function to Q, i.e., the function r 7! e2πiry; is a nontrivial character of Q. When y = 0, this is the trivial character. It is easy to believe that these may be all of the characters of Q, since the usual picture of Q inside R makes it hard to think of any way to write down characters of Q besides those of the form r 7! e2πiry (y 2 R). However, such characters of Q are only the tip of the iceberg. We will now show how to make sense of e2πiy for p-adic y, and the new characters of Q which follow from this will allow us to easily write down all characters of Q. That is, in a loose sense, every character of Q is a mixture of functions that look like 2πiry r 7! e for y in R or some Qp. Technically, e2πiry is meaningless if y is a general p-adic number. We introduce a formalism that should be thought of as allowing us to make sense of this expression anyway. MATH 248A. THE CHARACTER GROUP OF Q 3 For x 2 Qp, define the p-adic fractional part of x, denoted fxgp, to be the sum of the negative-power-of-p terms in the usual p-adic expansion of x. Let's compute the p-adic fractional part of 21=50 for several p. In Q2; Q3, and Q5, 21 1 21 21 3 4 = +2+22+23+:::; = 2·3+32+36+:::; = + +2+2·5+:::: 50 2 50 50 25 5 Therefore 21 1 21 21 3 4 23 = ; = 0; = + = : 50 2 2 50 3 50 5 25 5 25 21 For any p 6= 2; 5, f 50 gp = 0. More generally, any r 2 Q is in Zp for all but finitely many p, so frgp = 0 for all but finitely many p. n m m Note that for 0 ≤ m ≤ p − 1, f pn gp = pn . In thinking of p-adic expansions as analogous to Laurent series in com- plex analysis, the p-adic fractional part is analogous to the polar part of a p-adic number. (One difference: the polar part of a sum of two mero- morphic functions at a point is the sum of the polar parts, but the p-adic fractional part of a sum of two p-adic numbers is not usually the sum of their individual p-adic fractional parts. Carrying in p-adic addition can allow the sum of p-adic fractional parts to \leak" into a p-adic integral part; consider 3=5 + 4=5 = 2=5 + 1.) There is an analogue in Q of the partial fraction decomposition of rational functions: X (1) r = frgp + integer: p This sum over all p makes sense, since most terms are 0. It expresses r as a sum of rational numbers with prime power denominator, up to addition by an integer. For example, X 21 21 21 = + 50 50 50 p p 2 5 1 23 = + 2 25 71 = 50 21 = + 1: 50 P To prove (1), we show the difference r − pfrgp, which is rational, has no prime in its denominator. For p 6= q, frgp 2 Zq, while r − frgq 2 Zq by P P the definition of frgq. Thus r − pfrgp = r − frgq − p6=qfrgp is in Zq. P Applying this to all q, r − pfrgp is in Z. The p-adic fractional part should be thought of as a p-adic analogue of the usual fractional part function {·} on R, where fxg 2 [0; 1) and x − fxg 2 Z. 4 KEITH CONRAD In particular, for real x we have fxg = 0 precisely when x 2 Z. The p-adic fractional part has similar features. For x 2 Qp, m fxg = 2 [0; 1) (0 ≤ m ≤ pn − 1); x − fxg 2 Z ; fxg = 0 , x 2 Z : p pn p p p p While the ordinary fractional part on R is not additive, the deviation from additivity is given by an integer: fx + yg = fxg + fyg + integer: This deviation from additivity gets wiped out when we take the complex exponential of both sides: e2πifx+yg = e2πifxge2πifyg. Of course e2πifxg = e2πix for x 2 R, so there is no need to use the fractional part. But in the p-adic case, the fractional-part viewpoint gives us the following basic definition: 2πifxgp For x 2 Qp, set p(x) = e . For example, 2πi(1=2) 2πi(23=25) 2(21=50) = e = −1; 3(21=50) = 1; 5(21=50) = e : 2πifx+ygp 2πifxgp 2πifygp The function p is a group homomorphism: e = e e . To see this, we have to understand the extent to which the p-adic fractional part fails to be additive. For x; y 2 Qp, x − fxgp; y − fygp; x + y − fx + ygp 2 Zp: Adding the first two terms and subtracting the third, we see that the rational number fxgp + fygp − fx + ygp is a p-adic integer. As a sum/difference of p-adic fractional parts, it has a power of p as denominator. Since it is also a p-adic integer, there is no power of p in the denominator, so the denominator must be 1. Therefore fxgp + fygp − fx + ygp 2 Z: Thus p is a group homomorphism.
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